According to a known technique of an ignition plug for igniting fuel in an internal combustion engine, a spark gap is formed between a center electrode and a ground electrode, and plasma is generated between the electrodes (refer to, for example, U.S. Pat. No. 4,568,855 “Patent Document 1” and Japanese Patent Application Laid-Open No. JP 2009-527078 “Patent Document 2”). Also, there is a known technique of supplying high-frequency electric power to the center electrode for generating plasma between the electrodes (refer to, for example, Japanese Patent Application Laid-Open No. JP 2008-529229 “Patent Document 3”).